1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods and apparatus for improving properties of solid materials by providing shock waves therein where the laser beam impacts the solid material at an oblique angle. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Known methods for the shock processing of solid materials, particularly, known methods for the laser shock processing of solid materials, typically used coherent energy from a laser beam oriented normal to a workpiece. This normal orientation, if shape of the laser beam is symmetrical, makes a symmetrical impact area on the workpiece.
In FIG. 1, laser shock processing is shown with the laser beam 10, having a particular diameter D, passing through a transparent overlay layer 12 to impact workpiece 14. The impact area of workpiece 14 is shown as the area of a spot having a diameter S1, shown in FIGS. 1A and 1B.
Particular constraints of laser shock processing are sometimes created based on the shape of the material or other geometric factors such as when attempting to work integrally bladed rotors (IBR""s), blind bores, slots, or dovetail sections. In these cases, the laser beam 10, from necessity, needs to impact workpiece 14 at a particular angle "THgr". This causes the dimensions of the impact spot to change in one or more directions. As shown in FIGS. 2a and 2b, the shape of the spot changes to an ellipse with a major diameter S2, even though the nominal beam diameter D of laser beam 10 has not varied.
The consequences of such a change of the incident spot shape necessarily changes the energy density applied to the workpiece 14, all other factors being constant. This particular change of the applied pressure, i.e., the energy density per unit area compared to other areas on the same surface creates a possibility of non-uniformly working the material, thereby possibly losing some of the benefits of laser shock peening.
Typical laser shock processing techniques and equipment can be found in U.S. Pat. No. 5,131,957 to Epstein, along with that of U.S. Pat. No. 5,741,559 entitled LASER PEENING PROCESS AND APPARATUS, assigned to the assignee of the present invention and hereby incorporated by reference.
This non-uniformity of energy application to a workpiece may cause severe problems, particularly when hitting a workpiece from opposite sides at the same time, as used with a split beam laser system. Such opposing hits are sometimes needed on workpieces of thin cross-section, such as disks, blades, and other workpieces of different geometries. In conventional split beam processing, there is a possible effect of not having the laser processed portions on the opposite sides of the workpiece worked identically, and at the same time when elliptical spots are utilized. Such non-uniform working of the workpiece may cause over or under working of the material, or distortion of the work pieces, thereby not achieving the goals of laser shock processing.
Furthermore, based upon the oblique angle "THgr", along with the particular transparent overlay material utilized, polarization issues regarding the reflection of the laser beam from the surface of the transparent overlay layer can possibly degrade and reduce the energy applied to the workpiece.
What is needed in the art is a way to modify the incident spot of laser applied energy to consistently make and have a uniform working of the workpiece at such location.
The present invention provides a method of laser shock processing that can be used in a production environment to significantly reduce the non-uniformity of the applied energy to a workpiece by modifying the shape and location of the applied laser energy pulse.
In one form of the invention, the method calls for applying the transparent overlay material to the workpiece and directing a pulse of coherent energy through the transparent overlay to contact the solid material at an oblique angle to create a shockwave. Such oblique angle may be greater than 10 degrees, even to between 60 to 80 degrees from a normal axis into the workpiece through the point of contact. Additionally, the laser processing method may comprise a step of passing the pulse of energy through a lens to reform the shape of the incident area on the workpiece, to counteract geometric effects created by the workpiece surface orientation on the incident area shape. In one form of the invention, the lens may be a cylindrical lens. Alternatively, other forms of lenses may be used, depending on the geometry of the workpiece, laser beam, and processing conditions utilized.
In another form of the invention, the transparent layer may be of non-uniform thickness, thereby acting as the lens.
In yet another form of the invention, the laser processing method could be used to process substantially opposite sides of the workpiece. Such method includes the steps of applying a laser beam substantially normal to the workpiece; applying a laser beam substantially oblique to the workpiece on a substantially opposite side thereof; and controlling the laser beam so the workpiece experiences substantially identical conditions on substantially opposite sides of the workpiece. This independent orientation of the pressure pulses and worked areas on the workpiece allows controlled interaction of the indepth worked areas on the workpiece. The method further includes processing methods, which counteract the geometric effects of such oblique angle to maintain the energy per unit area applied to the workpiece, and preferably make the energy applied to opposite sides of the workpiece substantially equal.
Another embodiment of the present invention, includes the steps of applying a transparent overlay material to the workpiece and directing a pulse of coherent energy to contact the solid material throughout the transparent overlay material to create a shockwave. Controlling the shape of a contacting pulse is based on the incident angle of the pulse to the workpiece. The method may further include a means for changing the polarization of the laser beam pulse based on the incident angle to the workpiece.
In another form of the invention, two laser beams are utilized to process substantially opposite portions of a workpiece. The laser beams are offset, (not co-linear) from one another to form pressure pulses on opposite sides of the workpiece that are substantially congruent and overlap each other as they pass through the workpiece.
The invention also includes an apparatus for improving the properties of the workpiece including the transparent overlay applicator for applying a transparent overlay to the workpiece, and having a laser associated with the transparent overlay applicator to provide an oblique laser beam through the liquid transparent overlay to create a shockwave on the workpiece. A positioning mechanism is attached to selectively position the workpiece relative to the laser beam, while a control unit is operatively associated to the laser beam and positioning mechanism to control the operation timing of the laser and selective operation of positioning mechanism. A means is included for changing the polarization of the laser beam supplied from the laser as a function of the relative position of the laser beam and workpiece.
Another embodiment of the present invention, includes an apparatus having a transparent overlay applicator for applying the transparent overlay to a workpiece, laser operationally associated with the transparent overlay applicator to provide an oblique laser beam through the transparent overlay to create a shockwave on the workpiece. A means for changing the shape of the provided laser beam, depending upon the relative angle between the laser beam and the workpiece surface, is provided, along with a control unit, operatively associated with the laser, to control the operation and timing of the laser and means.
An advantage of the present invention is that by modifying the spot shape by controlling the laser and laser beam shape, and/ controlling laser beam polarization to modify reflection, it is possible to control the shock pressure applied to the workpiece in a production environment.
Another advantage of the present invention is that it is now possible to process workpieces with a combination of normal laser beams and oblique laser beams from the same or different lasers. Further, the present invention processes such workpieces efficiently and uniformly.
Yet another advantage of the present invention is that the method provides for shaping the incident spot of contact, i.e., where the energy from the laser beam is incident upon the workpiece, to make a desired shape or energy concentration, depending upon the needs or features of the workpiece to be worked.
A further advantage of the present invention is that one can control the polarization of the laser beam relative to the incident angle of the laser beam to the workpiece. This control of polarization increases the efficiency of using the energy supplied from the laser beam, to cause more (or less) energy to couple with and work the workpiece.
Another advantage of the present invention is the fact that relatively thin workpieces such as airfoils may be processed with split beam processing with offset laser beams to create congruent pressure pulsed areas through the workpiece.